(shogi) A defense structure in Japanese chess in which the king (玉) is positioned in a certain way so that it is protected by pawns (歩) and silver general(s) (銀) and/or gold general(s) (金) often with an additional knight (桂) and lance (香車).

The castle was perhaps a figurative name for a close headpiece deduced from its enclosing and defending the head, as a castle did the whole body; or a corruption from the Old French word casquetel, a small or light helmet.

No. 24. ¶ If your adversary make a false move, castle improperly, &c., you must take notice of such irregularity before you move, or even touch a piece, or you are no longer allowed to inflict any penalties.